The present invention relates to an apparatus and method for making a post-foaming gel.
Before the present invention, attempts have been made to add foaming agents, commonly low pressure propellant mixtures, such as pentane/butane mixtures to gel bases in conventional partly-filled pressure vessels to make a post-foaming gel. However, any post-foaming gel of cosmetically acceptable stiffness will tend to foam spontaneously when exposed to atmospheric pressure if it contains bubbles of air or hydrocarbon. These act as nuclei for foaming, by expanding and shearing the gel in their immediate vicinity. This means that pentane/butane mixtures cannot be incorporated into gel base in a conventional, partly-filled pressure vessel, even when the headspace is pressurised with air or nitrogen. Inevitably some of the water-insoluble gas will be entrained in the gel. The resulting spontaneous foaming makes filling into cans very difficult and messy. Even then the product will not settle down on storage to give a satisfactory result.
Taking a potentially `easy` situation, such as adding only isopentane (B.Pt. 29.degree. C.) or n-pentane (B Pt 37.degree. C.), there are still problems. Assuming a good gel is produced by cold mixing, you will find that at temperatures around 20.degree. C. the mixing does vaporise significant amounts of foaming agent and the resulting gel is foamy.
These bubbly gels are not suitable for packing into cans because the external propellant used with the cans gives insufficient pressure to collapse the bubbles on storage. This is largely due to the fact that these gels have a yield value such that they resist the applied pressure (or fail to transmit the full effect to the bubbles).
A post-foaming gel is disclosed in U.S. Pat. No. 3,541,581. A continuous method and apparatus to make a post-forming gel is disclosed in U.S. Pat. No. 4,405,489. The filling of an aerosol can containing an interior plastic bag which holds the product to be dispensed is disclosed in U.S. Pat. No. 4,589,452.